Geolocating an RF transmitter in the presence of multipath interference presents a unique and difficult problem. Signals that do not arrive at a receiver via a direct pathway have an incorrect time, frequency (Doppler), and/or angle of arrival (AOA) compared to those corresponding to the direct path. As a result of this, an erroneous geolocation may be computed using these measurements. Also, errors introduced by multipath interference, for example from buildings, are generally not zero-mean Gaussian random processes and will not necessarily average out over multiple measurements.
Current geolocation systems measure time, frequency, and/or AOA measurements and compute a potential geolocation estimate from these measurements. The potential geolocation may have increased accuracy if the errors are zero-mean Gaussian and all of the transformations are linear.
Unfortunately, the above assumptions are not generally true in the presence of multipath interference. In the presence of multipath interference, the measurement errors are typically not zero-mean or Gaussian, and the transformations from time, frequency, and AOA are typically non-linear.
U.S. Pat. No. 5,999,129 to Rose discloses a multiplatform ambiguous phase circle and time difference of arrival protection emitter location system. More particularly, Rose describes determining the geolocation of a stationary RF signal emitter from two or more moving observer aircraft. The observers receive signals from the emitter, and the system measures the phase difference between the signals. The observers perform pulse time of arrival (TOA) measurements over a predetermined clock interval, and calculate the time difference of arrival (TDOA) of corresponding, same-pulse, emitter signals. Based on geometric relationships, the system creates a series of circular lines of position (LOPs) for each observer, and computes hyperbolic LOPs based on the TDOA calculations. The system determines the emitter location from the intersection of the hyperbolic LOPs and the circular LOPs.
U.S. Pat. No. 6,933,888 to Schiffmiller et al. discloses a multi-shop coherent geolocation system. More particularly, TOA measurements at a number of collecting platforms are performed with the positions of the platforms being ascertained. The measurements are sent to a geolocation computer to arrive at the potential location of the emitter.
U.S. Pat. No. 7,317,417 to Arikan et al. is directed to a system for detection and tracking of one or more targets. A first slice of a cross-ambiguity function of a transmitted signal and the reflection thereof are calculated.